Are you sure its overheating and not just a sensor problem? i.e. does water start boiling.. If you're constantly losing water then maybe Head problems or a failed pressure cap on your top up bottle. If not then, 1. check water pump -does the heater(s) pump out lots of hot air when set to maximum - and doesn't go cold after a short while 2. check thermostat, and radiator - when it overheats , is bottom radiator hose cold? If so blocked radiator or closed thermostat. 3. Check Fans come on when overheating - but this is normally not a problem when moving at speed.